The ability to structure and pattern silicon is important for many applications. Many devices require bulk micromachining, which defines structures by selectively etching deep inside the substrate. The process is enabled by modern photolithography in combination with tools and techniques that have been developed specifically for deep etching into silicon. Relevant information regarding silicon fabrication processes known to those of skill in the art can be found, for example, in Sami Franssila, Introduction to Microfabrication (John Wiley & Sons, 2004), and the references cited there.
Some of the bulk micromachining processes known in the art are dry and some are wet. For example, reactive ion etching (RIE) and inductively coupled plasma (ICP) are two dry techniques that utilize a combination of reactive ions and kinetically charged species to “mill” into a material and define high aspect ratio features. For all these methods, the depth of the trenches is limited by the slight taper in the sidewalls after the etch. The slight taper causes the trench to get narrower as the silicon wafer is etched deeper. The taper limits the resulting aspect ratio (which is the ratio of the maximum depth to the trench width), and effectively either limits the depth of the features or the minimum feature size. For deep RIE processes, the aspect ratios can be slightly better than 20:1.
Much attention has been devoted to optimizing and reducing the slight taper in RIE processes. Companies have developed methods for operating these tools in specific ways to decrease the taper, increase the aspect ratio, and ease the tradeoff between the maximum obtainable depth and minimum feature size; for example the Bosch process used by Oxford. These processes do improve the aspect ratio, however designers are still limited by the aspect ratio. In addition, most of the processes to improve the aspect ratio also result in rough sidewalls, which is non-ideal for many applications. Furthermore, RIE is expensive, low throughput, and requires a high vacuum to operate.
Wet processing is also used to etch high aspect ratio features into silicon. Potassium hydroxide (KOH) and tetramethylammonium hydroxide (TMAH) are two chemicals typically utilized in an aqueous environment to etch and structure silicon. This process takes advantage of the fact that silicon has a well-defined crystal structure and certain crystal planes are more susceptible to etching. The etch results in pits that have angled walls, with the angle being a function of the crystal orientation of the substrate. This technique does not require high vacuum systems thus it is less expensive than the dry processing techniques described above. However, the crystal-dependent nature of the process requires additional consideration for wafer orientation and device design making processing less straightforward. In addition, although these wet etches are highly anisotropic and have a preferred etch direction, they still do etch in the non-preferred directions. Thus, although these processes obtain high aspect ratio features, they are also limited by the sidewall taper. In general wet techniques have a larger aspect ratio and smoother sidewalls than dry techniques, but require fine alignment of the features with the crystallographic orientation of the wafer. In addition, since these techniques are limited to the specific crystallographic directions, only rectangles and lines (and not arbitrary shapes in the x-y plane) can be patterned using this technique.
There is thus a need for a process which allows etching of silicon at high aspect ratios but which improves on existing processes such as reactive ion etching.